Apparatus for forming one-piece metallic can bodies are known and typically comprise a reciprocating movable ram movable through a relatively long forward working stroke and a rearward return stroke via a mechanical drive having an output shaft driven in reciprocating straight line motion and connected to the non-working end of the ram to reciprocate same. Prior to commencement of the working stroke, a can blank having a cylindrical side wall and integral bottom wall is positioned, using known indexing means, in the path of the ram to engage the working end entering the can through its open top at the commencement of the working stroke. The working end drives the can blank at high speed through a precisely aligned die package. As the metallic can enters the die package, its bottom end first, a drawing action occurs followed by an ironing action. A redraw die reduces the can diameter as the can is pulled by the ram through the die opening. The ironing die thins the metal can side walls by squeezing the metal between two surfaces to lengthen the can body. A pilot die helps to minimize thickness variation in the irregular can edge by supporting the ram punch as the irregular edge passes through a middle ironing die. In the final steps of the process, at the end of the working stroke, the ram working end or punch strikes a bottom former to shape a concave dome in the can bottom. The foregoing die pack comprising drawing and ironing dies, a pilot die and bottom former are conventional and may be arranged in various configurations. Critical, however, is the requirement that the die package be perfectly centered on the ram drive axis with precise tolerances between the ram and die package.
The reciprocating ram is typically supported in a mechanical rolling element bearing support or a hydrostatic type bearing support assembly wherein the ram movements are supported on constantly pressurized and flowing, thickened bearing oil films for maximum movement accuracy and minimum wear. At reduced speeds of ram movement such as during commencement and termination of the working and return strokes, the hydrostatic type bearing supports generally provide sufficient hydrostatic type support since the pressurized oil films supplied to the outer surface of the ram through hydrostatic pressure pad slots formed in the bearings through which the ram moves are capable of flowing circumferentially about the ram so as to center the ram within the bearing. However, at high speeds of ram movement during the working and return strokes, this circumferential flow is somewhat disrupted by the high axial component of velocity of the ram moving through the bearings. Therefore, at relatively high ram speeds during working operations, there may be a partial or complete failure in the ability of the bearings to provide hydrostatic bearing support which may result in complete bearing failure or early bearing fatigue necessitating frequent replacement or repair of the bearings. Such bearing fatigue also adversely effects the ability of the bearings to support the ram during its reciprocal movements while maintaining repeated trueness of movement with minimum wear for maximum quality of finished can bodies.
Another problem associated with hydrostatic type bearing support systems for a reciprocal ram of which I am aware is that the unsupported, overhanging working end portion of the ram (i.e., which extends outwardly from the bearing support and therefore only supported at the bearings) is the natural tendency of the working end to vertically deflect due to its own weight, with the amount of downward deflection due to gravity of the overhanging portion varying with the fourth power of the length of overhang. In other words, the vertical deflection of the ram working end increases as the ram progresses in its working stroke towards the die package. In multiple stage can drawing and ironing operations wherein the ram must be extended or projected within the dies over a long unsupported distance, such large vertical deflections cannot be tolerated if the necessary trueness, quality and metal wall constant thickness and completed part surface finish is to be maintained. Heretofore, in the prior art of which I am aware, this problem of natural vertical deflection due to inadequate ram bearing support has either been an operational speed limiting factor if frequent failures and maintenance interruptions for bearing replacements are to be avoided, or has resulted in limiting the length of the working stroke and therefore the length of can bodies that may be manufactured with such a drawing and ironing process.
A known mechanical drive of which I am aware for reciprocating the ram in straight line high speed movement in a horizontal plane generally comprises a crank shaft, rotating in a bearing set, and formed with a throw at one end thereof to which is connected a connecting rod driven by the crank shaft. The horizontal drive axis of the crank shaft is located below the horizontal plane of movement of the reciprocating ram. A pivoting beam is connected to the other end of the connecting rod at an intermediate portion of the rod and below the horizontal plane of ram movement. The lower end of the vertically extending pivoting beam is pinned to a machine base and the upper end of the beam in the horizontal plane of ram movement is connected to the ram with a connecting link. The ram is constrained to move horizontally via a slide way.
In the known crank drive described above, the only rotating part is the crank shaft; all the other aforesaid parts reciprocate. Balancing of this mechanism is accomplished with additional throws to move additional connecting rods and beams in a direction opposite to that of the ram. All of these balancing parts also reciprocate.
Since the reciprocating parts of the crank drive mechanism are located at the distal end of the crank shaft remote from the support bearing set, substantial cantilevering forces result which cause the crank shaft and reciprocating mechanism to experience internal bending transmitted to the output shaft; this causes the ram to deflect from its straight line motion, particularly within the horizontal plane. Further, since the mechanical parts of the prior art crank drive are subject primarily to reciprocating movement and not rotational movement (only the crank shaft rotates), the amount of input energy that must be dissipated each time the reciprocating parts reverse their direction at each end of the stroke is large and, since the input energy is dissipated through an external flywheel, the flywheel size is resultingly great, i.e., the rotating inertia of the flywheel is approximately three times as great as the flywheel used in the present invention.